Sterile Petri Dishes in a Clinic Lab: Handling, Labelling and Incubation Basics

September 14, 2026

Graduated specimen container with lid used at a clinic laboratory bench

Sterile Petri Dishes in a Clinic Lab: Handling, Labelling and Incubation Basics

The dish arrives sterile and the sample does not. Everything that happens between those two facts decides whether the result is a colony count a clinician can act on or a plate of mould with no interpretation. For a clinic-sized laboratory, the routines around the dish matter more than the dish itself.

What the Dish Provides

A 100 × 15 mm sterile polystyrene dish is a single-use container holding a thin layer of agar. Polystyrene is optically clear, which matters for colony counting on a light box, and it is not intended for repeated autoclaving: the plastic warps at sterilization temperature. Dishes are supplied sterile in sleeves of ten and cased 500 to a case for a reason, because opening a sleeve exposes every dish in it.

Graduated specimen container with lid used at a clinic laboratory bench

Handling Without Recontaminating

Open the sleeve only when plates are about to be poured or used, work on a clean surface, and keep the lid over the agar whenever the plate is not actively being worked on. Lift a lid edge rather than removing it and setting it down; a lid placed face-down on the bench picks up organisms that then transfer to the agar. Do not stack plates more than a few high while setting, and let agar solidify fully before moving the stack, because sliding a warm plate spreads condensation across the surface.

Labelling That Survives the Incubator

Label the base, not the lid. The lid rotates; the base stays with the agar, and a base label keeps the plate identity attached to the culture if lids get mixed during reading. Write the sample identifier, the date and the site or medium on the base with a marker that resists condensation, and keep the identifier short enough to read through the plate lid on a light box. Carry the same identifier into the laboratory information record so a plate photographed during reading can be traced back to the request.

Incubation Basics

Incubate plates inverted, with the agar up, so condensation that forms on the lid drips onto the lid rather than onto the growth surface. Clinical bacteriology laboratories typically hold plates around 35 ± 2°C for routine cultures, while environmental and fungal work uses lower temperatures, and the request or the method sheet should state which applies. Keep the incubator loading even, do not stack to the point where airflow is blocked, and record which plate went in when, because incubation time is part of the result.

Laboratory quality requirements are set out in ISO 15189, which covers competence, quality and quality control for medical laboratories and expects documented procedures for specimen handling from receipt onward. Public Health Ontario's infection prevention and control library is a practical starting point for a clinic laboratory writing those procedures. A small clinic laboratory that writes down its plate-handling routine, its incubation temperature and its labelling convention is already working to that structure.

Plate, Tube and Container Together

Specimen containers and lids staged for laboratory sample handling

Dishes rarely travel alone. A specimen arrives in a sterile screw-cap container, is divided into 15 mL tubes for processing, and is then plated. Matching the container to the test at the point of collection is what keeps the plate from being voided at the bench.

Further reading for laboratory teams is collected in the learning hub, alongside microbiological monitoring for labs and the six sample rejection failure modes.

Frequently Asked Questions

Should petri dishes be labelled on the lid or the base?

On the base. The lid rotates and can be swapped between plates, while the base stays with the agar and the growth. Put the sample identifier, date and medium on the base with a condensation-resistant marker.

Why are plates incubated upside down?

Inverting the plate keeps condensation that forms on the lid from dripping onto the agar surface, which would spread colonies and blur the growth pattern. Incubate agar side up.

Can polystyrene dishes be autoclaved and reused?

No. Polystyrene warps at autoclave temperatures. These dishes are supplied sterile for single use, and reuse also breaks the traceability chain between sample and plate.

What temperature does a clinic laboratory incubate plates at?

Routine clinical cultures are typically held at 35 ± 2°C, with environmental and fungal work at lower temperatures depending on the method. The procedure sheet for the test in question sets the number.

Last updated: September 2026. CliniEco Medical is a licensed medical device establishment (MDEL #35334).